JP2007519432A - System and method for distortion reduction in an electromagnetic tracker - Google Patents

Abstract

Disclosed is an analysis and reduction of distortion in an electromagnetic (EM) tracker.
EM trackers can use coils as receivers and transmitters. Particular embodiments of the system (100) include a tracking analysis unit (120) that analyzes the tracking behavior of the instrument (110) and a tracking correction unit (130) that corrects the tracking behavior of the instrument (110). Such an instrument (110) is a medical instrument such as a drill, a catheter, a scalpel or a scope. These instruments and their surrounding environment often include metal components. Placing an EM navigation device on the instrument (110) such as a receiver or transmitter can cause distortion in the magnetic field that affects tracking and tracking accuracy. Uses other than medical applications are anticipated, and tracking systems other than EM tracking systems such as ultrasound or inertial position are also anticipated.
[Selection] Figure 1

Description

  The present invention relates generally to electromagnetic tracking systems. Specifically, the present invention relates to a system and method for reducing distortion caused by tools and other elements in an electromagnetic tracking system.

  Physicians such as physicians, surgeons and other medical professionals often rely on techniques such as image guided surgery or examination when performing medical procedures. The tracking system can provide, for example, medical device position information relative to a patient or a reference coordinate system. The doctor can refer to the tracking system when the medical instrument is not in the doctor's line of sight to determine the position of the medical instrument. The tracking system can also help in pre-surgical planning.

  A tracking or navigation system allows the physician to visualize the patient's anatomy and track the position and orientation of the instrument. The physician can use the tracking system to determine when the instrument has been positioned at the desired location. The doctor can perform the operation by positioning at a desired site or an injured site while avoiding other tissues. Increasing the accuracy with which medical instruments are placed inside a patient can improve the control of smaller instruments that have less impact on the patient and allow less invasive medical procedures. Improved control and accuracy of smaller and more precise instruments can also reduce the risks associated with more invasive procedures such as open surgery.

  The tracking system can also be used to track the location of items other than medical devices in various applications. That is, the tracking system can be used in other situations where it is difficult to accurately determine the position of the object or the instrument within the environment by visual inspection. For example, tracking techniques can be used in criminal investigations or security applications. Retailers can use tracking technology to prevent theft of goods. In such a case, a passive transponder can be installed on the product. Transmitters can be strategically installed in retail establishments. The transmitter sends an excitation signal at a frequency designed to generate a response from the transponder. When a product having a transponder is located within the transmission distance of the transmitter, the transponder generates a response signal that is detected by the receiver. The receiver then determines the position of the transponder based on the characteristics of the response signal.

  Tracking systems are also often used in virtual reality systems or simulators. The tracking system can be used to monitor a person's location within a simulation environment. One or more transponders can be installed on a person or object. The transmitter sends an excitation signal and the transponder generates a response signal. This response signal is detected by the receiver. The signal sent by the transponder can then be used to monitor the position of people and objects in the simulation environment.

  The tracking system can be, for example, an ultrasound, inertial position or electromagnetic tracking system. Electromagnetic tracking systems can use coils as receivers and transmitters. Typically, electromagnetic tracking systems are constructed with an industry standard coil architecture (ISCA). ISCA uses three co-located orthogonal quasi-dipole transmitter coils and three co-located quasi-dipole receiver coils. In other systems, three large non-dipole non-co-located transmitter coils can be used with three co-located quasi-dipole receiver coils. Another tracking system architecture uses an array of six or more transmitter coils that are spaced apart and one or more quasi-dipole receiver coils. Alternatively, a single quasi-dipole transmitter coil can be used with an array of six or more receiver coils spaced apart.

  The ISCA tracker architecture uses a 3-axis dipole coil transmitter and a 3-axis dipole coil receiver. Each three-axis transmitter or receiver is configured such that the three coils exhibit the same effective area, are oriented orthogonally to each other, and converge at the same center point. If the coil is sufficiently small compared to the distance between the transmitter and receiver, the coil can exhibit dipole operation. The magnetic fields generated by the three transmitter coils can be detected by the three receiver coils. For example, nine parameter measurements can be obtained using three substantially concentric transmitter coils and three substantially concentric receiver coils. From nine parameter measurements and one known position or orientation parameter, position and orientation calculations can determine the position and orientation information of each of the transmitter coils for three receiver coils with three degrees of freedom. .

  Many medical procedures include medical instruments such as drills, catheters, scalpels, scopes, shunts or other tools. Many instruments used in medical activities include metal components. Furthermore, the environment surrounding the medical device or tracking system may include metal. Metals or other such materials can distort the magnetic field of the electromagnetic tracking system. Distortion of the electromagnetic tracking system can cause the tracking system to be inaccurate.

For example, physicians rely on electromagnetic tracking systems to perform delicate image guided surgery. The accuracy of position measurement is important when guiding precision instruments into a patient without direct gaze. Distortion can cause inaccurate positioning and pose a potential risk to the patient.
US Pat. No. 5,847,976 US Pat. No. 6,516,213 US Pat. No. 6,369,564

  Thus, a system that reduces inaccurate tracking measurements would be highly desirable. A system that minimizes the effects of distortion on position measurements would be highly desirable.

  Accordingly, a need exists for a system and method for reducing distortion caused by tools and other elements in an electromagnetic tracking system.

  Certain embodiments of the present invention provide systems and methods for distortion analysis and reduction in electromagnetic trackers. Certain embodiments of the improved electromagnetic tracking system include a transmitter and receiver that track the object and a distortion processing system that analyzes the distortion characteristics of the object. The transmitter transmits a signal and the receiver receives a signal from the transmitter.

  In an embodiment, the distortion processing system analyzes the signal received by the receiver to determine tracking accuracy. The distortion processing system can include a virtual electromagnetic tracker that simulates the effect of distortion characteristics on tracking. The strain characteristic can be a magnetic field. The distortion processing system can coordinate the tracking operation of the object.

  Certain embodiments of the strain processing system include a tracking analysis unit that analyzes instrument tracking behavior and a tracking correction unit that corrects instrument tracking behavior. The tracking correction unit may correct the instrument tracking behavior by adjusting the instrument tracking behavior. The tracking correction unit can also correct the tracking behavior of the instrument by adjusting the tracking system that tracks the instrument. The tracking analysis unit can test the adjusted tracking behavior of the instrument. In an embodiment, the tracking analysis unit generates a map and / or model of instrument distortion characteristics. The strain characteristic can be a magnetic field. In an embodiment, the distortion processing system is a computer simulation distortion processing system.

  Certain embodiments of the virtual tracking system include an object simulation module that simulates an object to be tracked and a simulation toolset that analyzes at least one distortion characteristic of the object. The simulation toolset can generate target distortion information based on the distortion characteristics. In an embodiment, the simulation toolset includes an accuracy module that determines the accuracy of the target simulation tracking. The simulation toolset can include a distortion detection module that determines the effect of distortion by the object on tracking accuracy. The simulation toolset can also include a strain modeling module that generates an electromagnetic model for evaluating the effect of the subject's strain on the tracker environment. Furthermore, the simulation toolset can include a distortion correction module that improves the distortion tolerance of the electromagnetic tracker.

  Certain embodiments of a method for distortion analysis in an electromagnetic tracking system include measuring an object tracking behavior and analyzing the object tracking behavior to determine the effects of distortion. The step of analyzing may further include generating a magnetic field map and / or model of the object for analyzing the tracking operation of the object to determine the effect of distortion. The method may also include simulating object tracking to determine the effects of distortion. Further, the method can include modeling the electromagnetic field in the electromagnetic tracking environment to analyze the effects of distortion. The method may also include the step of improving the distortion tolerance of the electromagnetic tracking system based on the effects of distortion.

  In an embodiment, the method includes adjusting the tracking behavior of the instrument to reduce the effects of distortion. The method can further include testing the instrument to confirm a reduction in the effects of distortion. In an embodiment, the method includes adjusting an electromagnetic tracker to correct for distortion effects. The method may further include the step of testing the electromagnetic tracker to confirm a reduction in distortion effects.

  The foregoing summary, as well as the following detailed description of specific embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, certain embodiments are shown in the drawings. However, it should be understood that the invention is not limited to the arrangements and instrumentality shown in the attached drawings.

  For purposes of illustration only, the following detailed description describes an embodiment of an electromagnetic tracking system for use with an image guided surgical system. It should be understood that the present invention can be used in other imaging systems and other applications.

  FIG. 1 illustrates a distortion processing system 100 used to improve the accuracy of position measurement of an electromagnetic (EM) tracker used according to an embodiment of the present invention. The system 100 includes an instrument 110, a tracking analysis unit 120, and a tracking correction unit 130. The tracking analysis unit 120 monitors the tracking operation of the instrument 110. The tracking correction unit 130 attempts to improve or correct the tracking operation of the instrument 110 based on information from the tracking analysis unit 120. The tracking analysis unit 120 and the tracking correction unit 130 can be implemented in hardware and / or software as separate units or as a single unit. The tracking analysis unit 120 and / or the tracking correction unit 130 can be integrated with the EM tracker or can be a separate system.

  The instrument 110 can be any instrument used in medical activities, such as an orthopedic instrument (eg, an electric or pneumatic drill), a catheter, a scalpel, a scope, or other instrument. The instrument 110 may generate or affect a magnetic field that causes distortion in the EM tracker measurements. Placing an EM navigation device, such as a receiver or transmitter, on the instrument 110 can have distortion and / or distortion effects on tracking.

  The tracking analysis unit 120 analyzes the tracking operation and the effects of the instrument 110. The tracking analysis unit 120 performs actual and / or simulated EM tracking of the instrument 110 to determine the effect of distortion by the instrument 110 on the position and / or orientation calculations. The tracking analysis unit 120 may be a computer that simulates the effects of the instrument 110 in the tracking system. Receivers, transmitters and / or other sensors located on the instrument 110 can be used to collect information regarding the instrument 110. The tracking analysis unit 120 acquires the magnetic field data of the instrument 110. The tracking analysis unit 120 generates position and orientation data of the instrument 110 in the tracking coordinate system. Other effects on the tracking of the instrument 110 can be measured and / or simulated by the tracking analysis unit 120. The tracking analysis unit 120 can also generate a map and / or model for the field and distortion effects around the instrument 110.

  The tracking correction unit 130 adjusts or corrects the tracking operation of the instrument 110. The tracking correction unit 130 uses maps, models and / or other data from the tracking analysis unit 120 to minimize the effects of distortion by the instrument 110. The tracking correction unit 130 can test different receiver and / or transmitter configurations for the instrument 110 to improve the tracking operation of the instrument 110. The tracking correction unit 130 can correct, recalibrate, or reprogram the EM tracker to offset the effects of distortion due to the instrument 110.

  The tracking operation of the instrument 110 can be adjusted or corrected in various ways. For example, a mathematical model of components of instrument 110 and / or system 100 can be developed to correct for distortion. For example, a magnetic field model can be developed and corrected to reduce errors in tracking the instrument 110. Alternatively, distortion can be modeled and errors corrected, such as by using a ring model of the instrument 110 distortion after tracking measurements. Furthermore, the instrument 110 or another system 100 component can be shielded to create a known distortion that can be corrected before or after the tracking measurement. The EM tracker can be calibrated and / or the transmitter or receiver position of the instrument 110 can be adjusted to correct or reduce tracking distortion.

  In another embodiment, a strain generator adjacent to the instrument 110, such as adjacent to a three-coil transmitter or receiver on the instrument, is corrected by characterizing the instrument 110 that includes the strain generator. be able to. Alternatively, rather than accurately characterizing the transmitter, it is possible to track the transmitter gain using, for example, a one-coil transmitter and receiver array. Tracking gain helps to reduce the effects of distortion in tracking instrument 110.

  In an embodiment, the EM tracker includes a transmitter that transmits a signal, a receiver that receives the signal from the transmitter, and tracker electronics that analyzes the signal received by the receiver. Tracker electronics can be configured in software. The tracker electronics determines the position and / or orientation of the instrument 110 in the tracking coordinate system based on information from the receiver and / or transmitter. In an embodiment, the receiver is positioned on the instrument 110 to determine the position and / or orientation of the instrument 110 relative to the transmitter. In another embodiment, the transmitter can be installed on the instrument 110 to determine the position and / or orientation of the instrument 110 relative to the receiver. The EM navigation device used in the EM tracker can be, for example, a wired and / or wireless device (such as a wireless transmitter). The configuration of the EM tracker can be adjusted using information from the distortion processing system 100 to correct for the effects of distortion due to the instrument and operating environment.

  In operation, the instrument 110 is tracked using an EM tracker or other EM navigation device. The instrument 110 can be tracked with a physical EM tracker, or a simulation of the instrument 110 can be tracked with a virtual tracking system. The tracking analysis unit 120 measures parameters such as field, position and orientation data in connection with tracking the instrument 110. The tracking analysis unit 120 generates a distortion model and / or field map of the instrument 110. The tracking correction unit 130 then uses the model and / or map and additional data from the tracking analysis unit 120 to minimize the effects of distortion by the instrument 110. The tracking correction unit 130 can test various receiver configurations and / or placements for the instrument 110 to minimize EM tracker distortion. For example, by placing the receiver assembly at a particular location on the drill, the effects of distortion caused by the drill field can be minimized. The tracking correction unit 130 can also reconfigure or recalibrate the EM tracker to take into account the effects of distortion due to the instrument 110. For example, the EM tracker can be programmed to pre-process certain distortions caused by the tracking drill. In addition, the tracking and correction unit 130 can program the EM tracker to ignore the effects of environmental distortions such as metal on patient positioning surfaces, lights and / or room structures. The tracking analysis unit 120 can then test the instrument 110 of the EM tracker to confirm position accuracy. If the test is satisfactory, the physician can track with the EM tracker using the instrument 110.

  FIG. 2 shows a flow diagram of a method 200 for distortion processing of an electromagnetic tracking system to use, according to an embodiment of the present invention. First, in step 210, the instrument 110 is captured. In embodiments, the instrument 110 can be captured physically or virtually (eg, with an image or electronic display).

  Next, in step 220, the tracking motion of the instrument 110 is measured. For example, if the instrument 110 is physically captured, the magnetic field, position, orientation and / or other data is measured. If the instrument is displayed virtually, the effects of the instrument 110 can be simulated, for example, on a computer or virtual tracker. Distortion and / or other characteristics of the instrument 110 during electromagnetic tracking can be monitored and / or simulated. A map and / or model can be generated to analyze the effects of magnetic fields and strain around the instrument 110.

  Next, in step 230, the tracking operation of the instrument 110 or tracker is adjusted to reduce or correct the effects of distortion by the instrument 110. Errors in tracking the instrument 110 are analyzed. Various instrument 110 and receiver assembly configurations can be tested to determine which configuration minimizes distortion effects and other errors. The tracker calibration or configuration can be tested to determine which configuration best corrects distortion and other errors.

  Furthermore, residual errors remaining after adjusting the instrument 110 or tracker can be analyzed. The residual error can be analyzed to determine whether the residual error is below a certain threshold. For example, if the residual error is sufficiently small, the residual error can be ignored. In addition, the stability of the residual error is analyzed. The residual error can be analyzed to determine whether the residual error is reliably below a certain threshold of, for example, given noise and other factors. In embodiments, if the residual error is not reliably below a certain threshold, the instrument 110 or tracker is further adjusted to reduce the residual error.

  Next, in step 240, the adjusted instrument 110 and / or tracker is tested to verify tracking accuracy. In an embodiment, the location measurements are reacquired using the maps and / or models generated during step 220 described above. If the tracking data of the instrument 110 meets certain criteria or improvement levels, the instrument 110 can be used with an electromagnetic tracker.

  Many methods can be used to evaluate the electromagnetic behavior of the instrument 110. FIG. 3 shows a flow diagram of a method 300 for electromagnetic assessment of the instrument 110, according to an embodiment of the invention. First, at step 310, a preliminary test can be performed to estimate an optimal receiver assembly placement for the instrument 110 to be tested. Next, in step 320, the receiver placement is evaluated over the operating range values through accuracy considerations. Next, in step 330, a workflow test is performed to address the workflow issue. Workflow tests can be performed at various levels of detail to determine the interaction between the instrument 110 and the surgical workflow. The examples shown in the steps of method 300 are further described below.

  For example, a whiteboard or robot-based preliminary test is used to estimate the optimal EM receiver pack placement for the instrument 110 under consideration. The analysis of the suitability of the receiver arrangement can be performed, for example, by analyzing the tracking system suitability data for the position of the instrument 110. In addition, analysis of the stability of position and orientation data across repeated measurements can be performed on fixed EM receiver and transmitter positions.

  The placement of the EM receiver pack can be evaluated by robot and manual model accuracy analysis. The EM receiver pack can be attached to the instrument 110 below the test and / or robot evaluation fixture. A robot and / or manual model can be used to evaluate the associated error over the operating range of the EM tracker for a given tip offset length. For example, the maximum error value, root mean square (rms) value, standard value, and histogram can be analyzed for multiple measurements. Furthermore, an operation range with allowable accuracy can be acquired. Also, the effect of the operation of the actuation motor on the instrument 110 can be analyzed. Furthermore, the ability of the electromagnetic field integration detector (FID) to detect position errors due to metal distortion of the instrument 110 can be analyzed.

  Various workflow tests can be used to address the workflow aspects of the electromagnetic tracker and distortion processing system 100. The impact of the travel range and patient environment is measured based on out-of-range positions and FID warnings. In an embodiment, the instrument 110 is represented by a rough approximation. For example, a surgical drill is represented by a spray gun. The drill guide can be represented by, for example, a wooden dowel with a receiver pack taped in place. Tests using rough approximations are intended to provide an initial indication of the interaction between the instrument and the surgical workflow for various applications.

  In another embodiment, the test is performed with a closer approximation of the instrument 110. Appropriate materials are used to identify tracking and FID errors. The test can simulate the patient environment by using, for example, stainless steel sheets and / or bars bolted on a wooden test table as an alternative to the operating room and fracture table.

  In another embodiment, prototype equipment can be used to closely mimic the instrument 110 in clinical testing. The prototype instrument model can be used on a cadaver or subject in a patient environment such as a hospital operating room. Tracking and FID errors can be identified using tests.

  In addition, a simulation toolset can be used to develop and test an EM tracker system. The simulation toolset can investigate, for example, the tracking performance, metal resistance and / or strain detection of different coil architectures. The tool set can be adjusted to develop and test the function of the instrument 110 and / or the application of the instrument 110.

  A simulation tool can be used to examine the accuracy limits of an undistorted tracker. A virtual tracker (VT) with a simulation toolset provides a control system for examining accuracy limits due to signal-to-noise ratio, assembly error, calibration error, and other factors. Individual tracker parameters can also be considered for individual and / or combined effects on tracker system accuracy.

  Simulation tools can also be used for distortion modeling. In an embodiment, the VT includes electromagnetic modeling of the physical environment. This model can be used to analyze the effects of subject distortions in or near the tracker's operating volume. Objects that generate distortion can include, for example, surgical tools, room structures (eg, rebar floors and / or lead walls), and operating room tables or lights. First, the distortion characteristics of the object are evaluated for a given coil architecture. In the embodiment, distortion characteristics are evaluated without a correction technique. VT is used to derive a sensor architecture that is more resistant to metal strain, including constant strain (eg, of the surgical instrument being tracked) and accidental strain (eg, of a table or clamp).

  VT can also be used to improve distortion correction or distortion tolerance tracking. Strain modeling data can be used to promote strain tolerance of the EM tracking system. For example, the sensor can be integrated with a straining device by passive and / or active shielding techniques. Strain mapping techniques such as position and orientation (P & O) mapping or EM field mapping can be based on, for example, measurement data, simulation data, or a combination of data. By extracting reduced degree of freedom (DOF) parameterized models (eg, multiple dipole models) from measured and / or simulated data, distortion tolerance or correction can be improved. In embodiments, multi-sensor “mapping chambers” and / or robotic data collection can be used to map strain data.

  Furthermore, distortion detection can be improved. The FID algorithm can be improved. For example, the FID of each position sensor, the method of collecting and analyzing information by a plurality of sensors, the method of clarifying the vulnerability of the FID algorithm, and the model adaptation processing method can be improved.

  FIG. 4 shows a virtual tracker (VT) 400 for developing an EM tracker to use, according to an embodiment of the present invention. The VT 400 includes a target simulation module 410 and a simulation tool set 420. The VT 400 can be implemented as software in a general-purpose computer or a dedicated processor or circuit, for example. The VT 400 can be integrated with the distortion processing system 110 described above.

  Object simulation module 410 simulates the object or instrument 110 to be tracked. The target simulation module 410 generates data characteristics of the instrument 110. The simulation tool set 420 uses the simulation data from the target simulation module 410 to analyze the characteristics and tracking behavior of the instrument 110 within the virtual tracking system. In the embodiment, the simulation tool set 420 includes an accuracy module 430, a distortion detection module 440, a distortion modeling module 450, and a distortion correction module 460.

  The accuracy module 430 determines the accuracy of the EM tracker simulated by analyzing the instrument 110 or a virtual display of the instrument 110 and the effect of the instrument 110 on the accuracy of the EM tracker. The accuracy module 430 examines errors such as signal to noise ratio, instrument 110 assembly and calibration errors, and other factors to determine individual and / or combined effects on tracker measurement accuracy.

  The distortion detection module 440 determines the effect of distortion on the tracking accuracy. The strain detection module 440 determines the strain field due to the instrument 110 using field integration detection (FID), strain and instrument model, and sensor information. The distortion information from the distortion detection module 440 can be used to adjust the tracker and tracker software to improve tracking accuracy.

  The strain modeling module 450 generates an electromagnetic model of the EM tracker environment. This model is used to evaluate the effect of subject distortions in or near the tracker's operating volume. The strain modeling module 450 can model the strain field from various objects such as the instrument 110 and the environment (wall, floor, patient table, etc.) in which the tracker is operating. Information from the strain modeling module 450 can be used to develop a tracker sensor architecture that modifies the configuration of the instrument 110 and / or improves resistance to certain and / or accidental metal strains. .

  The distortion correction module 460 is used to improve the distortion tolerance of the EM tracker and related systems. The distortion correction module 460 may be used to simulate, for example, the effects of sensor shielding techniques (eg, passive and / or active), distortion mapping techniques, modeled object model extraction, and / or field mapping. it can. The simulated results and data can be used to program the EM tracker and / or improve the distortion tolerance of the EM tracker.

  In certain embodiments, VT 400 can be used to help achieve a tracking system that is robust against accidental distortion generators. Information from the VT 400 can help to allow close or ergonomic integration of the sensor with the surgical tool. Information from the VT 400 can help configure and calibrate the instrument, instrument guide and tracking system. The VT 400 module helps to ensure detection of conditions that cause inaccurate tracking of objects. Certain embodiments of the VT 400 may also enable a tracking system that operates with SNR limited tracking accuracy (eg, reduced system error).

  Various systems, methods and objects can be used for distortion detection, correction and tolerance according to certain embodiments of the invention. Some examples are described below for illustrative purposes.

  In the embodiment, when the distortion generation target (distortion generator) is attached to the EM transmitter, the distortion from the distortion generator can be mapped using various methods. For each transmitter coil of the transmitter, the vertical component of the magnetic field can be measured at the interface surrounding the strain generator. The field value without distortion can be subtracted from the measured value to determine the field of the strain generator at the boundary. In an embodiment, the Laplace equation and finite element analysis are used to calculate the field of the strain generator outside the boundary. In another embodiment, the strain generator is modeled as an array of dipoles of various positions, orientations and / or intensities within the boundary, for example. Adjust the position, orientation and / or strength of the dipole to fit the field at the boundary. The transmitter dipole is then added to the strain generator dipole array. These dipoles form an analytical model of transmitters and distortion generators outside the measured boundary. This model can be used to calculate the magnetic field, field square amplitude, and field gradient at any point outside the boundary.

  For the transmitter coil field amplitude, for example, a set of eight functions can be calculated from the square amplitude of the three transmitter coil fields. For example, a function can be established for each combination of x, y, and z component codes. For example, the function can be determined using table index interpolation and / or Raab's signal matrix code function. In an embodiment, the eight functions are equal if there is no distortion. By reversing the function, a distortion mapping can be created. In addition, the function can be rotated to create the desired mapping. A field and gradient module can be used to calculate a least squares optimal solution of the distortion.

  Alternatively, the signal matrix can be measured from the field of the strain generator. Next, the field amplitude of the transmitter coil is calculated. Determine the approximate position of the transmitter coil without distortion (eg, using the Raab algorithm). Select the transmitter coil sign and select a solution candidate. If the solution is close to the axis, the matrix function can be rotated away from the axis. A solution can be obtained. Select a sign. Next, the matrix is rotated back to the original position. The solution shows a solution with almost no distortion. Next, the least squares optimal solution is calculated using the calculated undistorted transmitter dipole. This solution includes distortion errors. A second least squares optimal solution is calculated using a field and gradient model with distortion. By iterating a distorted least squares fit, the results can be improved by second order effects.

  In an embodiment, the ISCA strain mapping process can be used to model strain generators as an array of dipoles. The strain-generating dipole is excited by the dipole field from the transmitter. The strain generator dipole gain is the ratio of the strain generator dipole moment to the undistorted transmitter field at the position of the strain generator dipole. The strain generator dipole is located in the direction opposite the transmitter field. However, the strain generator dipole may not be parallel to the transmitter field. Thus, the distortion generator gain can be a matrix or tensor, for example, since the distortion generator response will be different with respect to the undistorted transmitter field components in the x, y and z directions. In an embodiment, the gain matrix or tensor is independent of the transmitter field. As the transmitter moves, the distortion gain remains the same. The strain field can be recalculated using the new value of the transmitter field at the location of the strain generator. Thus, distortion correction can be tuned by several parameters and can be modified to suit the moved transmitter.

  For example, the receiver can be mounted on a debrider blade. The blade is mounted on the shaft in the debrider housing with an uncontrolled roll angle. The housing causes distortion. The housing is asymmetric so that the distortion varies with the roll of the housing relative to the receiver. A strain map with parameters that allow tracking time adjustment, such as the roll value of the map, can adjust the strain due to the debrider housing. By collecting additional data points, it is possible to calculate the roll value of the map to accommodate distortion generators fixed at either the transmitter or receiver or elsewhere. When there are more than one strain generators, the effects of each other strain generators can be addressed by iterative analysis. That is, for example, each distortion generator can be influenced by a field generated by a transmitter and a field generated by another distortion generator.

  In an embodiment, strain in the conductive ring can be modeled in the strain processing system 100. The position and orientation of the ring is established with respect to the transmitter. The shape of the ring is also determined. The cross section of the conductor is small compared to the size of the ring. Therefore, the conductor can be treated as a very small amount when the mutual inductance between the ring and the transmitter coil is calculated. Furthermore, the self-inductance of the conductor can be ignored when calculating the ring self-inductance. At this time, the self-inductance of the ring can be calculated from the shape of the ring. The mutual inductance between the ring and the transmitter coil can be calculated, for example, by Feynman's mutual inductance double integration. Using the transmitter model of the transmitter coil, the magnetic field per coil current for a particular position (x, y, z) can be determined in Tesla per ampere. A field model of the strain generator ring can then be calculated from the current value and the field per current value.

  In an embodiment, a transmitter coil board including a number of small helical coils can be used for magnetic field distortion mapping. The coils in the board are driven one by one by the drive board. In the embodiment, the time division coil on the coil board is driven by one frequency. The component side of the board faces the interior of the volume. The solder surface of the board faces the outside of the volume. Place the test device inside the volume. The board component surface is electrostatically shielded by a test device to form a drive current return path.

  Each coil on the board connects to a switch. The coil and the corresponding switch are connected in series. The non-selected coil is AC grounded. The current of the selected coil flows to the current measuring device. The switch can be, for example, an optocoupler, a diode or series of diodes, and / or a transistor. The Y line on the component surface is driven by an AC source. Each Y line is electrostatically shielded by a ground track on the component surface. The X line is located on the component surface and is AC grounded or virtual grounded. The X line is connected to a current measuring device. The coil is connected in series with the switch at the intersection of the X and Y lines. In an embodiment, the board includes a plurality of coils and switches connected in series at the intersection of a plurality of X and Y lines. The coil board or drive board may include time division logic that provides power to the coil. A time-division coil and a current measurement device are used to create a distortion mapping of the magnetic field around the test device.

  In an embodiment, the fluoroscopic imaging system uses a pair of ISCA receivers attached to a fluorescent camera calibrator. The fluorescent camera calibration tool can be attached to an image intensifier tube, for example. The image intensifier tube contains various metals and distorts the magnetic field from the electromagnetic tracker. A robotic system can be used to map the distortion and create a tracker correction table. This table can then be used to correct tracking errors due to distortion. Different image intensifier tubes of the same model exhibit slightly different magnetic field distortions. Therefore, each intensifier can employ a different distortion correction table.

  A distortion generating can can be used for the camera calibration tool. In an embodiment, the can includes a single piece of highly conductive metal such as copper, silver or aluminum. The can surrounds the image intensifier tube as much as possible, matches the mechanical constraints, and creates an X-ray transmission region in front of the image intensifier tube. The can also distorts the tracker's magnetic field. Therefore, the distortion caused by the can is mapped, and the distortion caused by the can is corrected using the created tracker correction table. In an embodiment, the metal thickness of the can is greater than the skin thickness of the metal so that the magnetic field does not penetrate so much that the metal and the volume behind the metal can be sensed. The can shields most of the magnetic field from the image intensifier so that the field of the intensifier is hardly distorted. Thus, by using a can, replacing the image intensifier tube has no appreciable effect on field distortion. One distortion correction table can be used for all image intensifier tubes of a given model. In another embodiment, one distortion correction table can be used for different models of image intensifier tubes.

  Further, fluoroscopy or other x-ray systems can include x-ray scattering filters. In an embodiment, the X-ray scattering filter includes alternating layers of lead and an X-ray transmissive material such as plastic or aluminum. X-ray openings can be provided that conduct electricity and thus block the magnetic field. The filter can block the X-ray opening of the conductive sheath that covers the image intensifier tube of the fluoroscopic apparatus. The sheath keeps the tracker's magnetic field away from the image intensifier, so that replacement of the image intensifier has minimal impact on the tracker.

  In an embodiment, a passive conductive ring can be placed in close proximity to the receiver of the fluoroscopy system. The passive ring can be arranged asymmetrically with respect to the ISCA transmitter, for example. The passive conductive ring defines the hemispherical area of the tracker system without a large winding coil. Alternatively, a hemispherical region can be defined by placing a ferromagnetic rod at one end proximate to the ISCA transmitter. Furthermore, the reciprocity allows a ring or rod to be placed close to the receiver instead of the transmitter to define the hemispherical region.

In an embodiment, an existing ISCA seed routine can be mapped so that a fixed strain generator seed can be calculated for the transmitter of system 100. For example, the square of the position component (x ² , y ² , z ² ) is calculated from the square of the sum of the mutual inductances of the three receiver coils for each transmitter coil. In an embodiment, the square of the inductance is independent of the receiver direction. If the strain generator is fixed with respect to the transmitter, a map can be calculated to replace the analytical generator's dipole solution. For the strain generator, the square of the inductance is again independent of the direction of the receiver. In addition, reciprocity allows the receiver and transmitter to be exchanged to determine position and orientation information for a strain generator that is fixed relative to the receiver.

  In embodiments, several data reduction methods can be used after the magnetic field mapping data is established. For example, Laplace's equations and finite element methods can be used at field map boundary points. The interior point can be used to check accuracy. Next, the distortion generator model can be expressed in several forms. This model can be, for example, a lookup table with interpolation, function fitness, and / or parameterized sum of basis functions. Basis functions can be selected for mathematical convenience and / or electromagnetic compatibility. For example, basis functions can be created for fields with parameterized magnitude strain rings, rods and / or sheets. The strain generator model can be expressed in terms of a computationally convenient function. Other basis functions can include, for example, dipoles of various lengths and orientations, and Green functions. In an embodiment, the electromagnetic sensitive basis provides a parameter value that results in a solution that satisfies the Laplace equation. In addition, a “good” model with relatively few parameters is created. Maps can be created from the model. The map group can perform distortion mapping of few data points.

  In an embodiment, a single dipole model can be created for a small conductive loop strain generator. The distortion generator is fixed with respect to the electromagnetic transmitter. This strain generator is assumed to be a perfect conductor. The field from the transmitter forms a magnetic flux through the loop. The loop includes an eddy current that forces the total magnetic flux through the loop to be zero. From the eddy current measurement, the strain field due to the loop can be calculated. For large loops, the strain field can be calculated using integration. For small loops, the loop can be approximated by a dipole and the eddy current field can be determined. For example, the area of the loop, the eddy current in the loop, and the magnetic flux through the loop can be used to determine the strain field due to the loop. In an embodiment, the distortion field can cause a change in transmitter current, which is measured and corrected by tracker electronics.

  In another embodiment, a single dipole model can be created for a small conductive object strain generator. The distortion generator is fixed with respect to the electromagnetic transmitter. It is assumed that this strain generator is a perfect conductor and does not generate magnetic flux inside the strain generator. The strain field of the target strain generator can be approximated by identifying a large cross section of the target in a plane perpendicular to the incident transmitter field. This cross section can be used to calculate the strain generation field as described above for the small conductive loop strain generator. In embodiments, the strain generator effective area is different for incident transmitter fields in different directions.

  In another embodiment, a small infinitely transmissive non-conductive ferrite object proximate to the transmitter can be analyzed. The object is preferably fixed relative to the transmitter. The object doubles or triples the incident magnetic flux inside the object, for example. This target strain field can be approximated as described above for a small conductive target strain generator. Next, the magnetic field into the object can be calculated by multiplying the strain field by a magnetic flux coefficient such as −2 or −3.

  Various instruments can be tested in the distortion processing system 100 and / or using the VT 400 to determine the effects of distortion on the tracking system and to calibrate the tracking system based on the effects of the distortion. For example, an air or electric drill can be tested to determine the effect of the drill on the position and orientation accuracy of the tracking system. Further, the strain processing system 100 can be used to estimate an appropriate mounting distance between the EM receiver and the drill. With proper receiver mounting distance, tracking of an instrument 110 such as a drill can be improved. The use of FID can improve the detection of false tracking due to magnetic field distortion by an instrument 110 such as a drill.

  To test the drill, the sensor is removed from the transmitter and placed parallel to the X axis of the tracking coordinate system. Attach the transmitter and receiver pack to the board. Collect position and orientation data in the absence of a drill to obtain calibration or reference data. Next, the drill is placed at a plurality of distances from the receiver pack. The orientation direction of the receiver pack relative to the upper surface of the drill is kept parallel for this plurality of distances. Collect position and direction for multiple distances. Tracker goodness of fit (GOF) data can also be recorded at multiple locations. Next, position information can be calculated, for example with respect to the tip of the drill. Use multiple test conditions to determine tip position error. FID data is calculated using navigation algorithms and thresholds. Since both the receiver pack direction and the tip offset were parallel to the X axis of the tracking coordinate system, the theoretical drill guide tube trajectory error can be estimated by obtaining the X component of the tip error. .

  Accordingly, certain embodiments of the present invention provide tracking systems and methods that are robust and resistant to accidental distortion. Certain embodiments provide ergonomic and efficient integration of EM position sensors with surgical tools or other instruments. Certain embodiments provide reliable detection of conditions that cause inaccurate tracking. Certain embodiments minimize system errors with tracking accuracy with limited signal-to-noise ratio. An electromagnetic tracking system and strain processor can be used to analyze and correct tracking errors due to metal distortion.

  Certain embodiments provide a virtual tracking system and simulation environment that can be used to measure distortion and other effects of tracking instruments, instrument guides and / or tracking systems and adjust their configuration. Certain embodiments provide systems and methods for minimizing distortion in a tracking system. Certain embodiments provide systems and methods for developing and testing strain resistant instruments and tracking systems.

  Although the invention has been described with reference to particular embodiments, those skilled in the art will recognize that various changes can be made and equivalents can be substituted without departing from the scope of the invention. Will. In addition, many modifications may be made to adapt a particular situation or material requirement to the teachings of the invention without departing from the scope of the invention. Accordingly, the invention is not intended to be limited to the specific embodiments disclosed, but is intended to include any embodiments falling within the scope of the claims.

The figure which shows the distortion processing system used in order to improve the position measurement precision of the electromagnetic tracker to be used by embodiment of this invention. 1 is a flow diagram illustrating a method of distortion processing for an electromagnetic tracking system to be used, according to an embodiment of the present invention. 1 is a flow diagram illustrating a method for electromagnetic evaluation of an instrument according to an embodiment of the invention. 1 is a diagram illustrating a virtual tracker for developing an EM tracker to use, according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Distortion processing system 110 Instrument 120 Tracking analysis unit 130 Tracking correction unit

Claims (11)

A transmitter that transmits the signal;
A receiver used to receive a signal from the transmitter and track an object with the transmitter;
A strain processing system (100) for analyzing strain characteristics of the object;
Improved electromagnetic tracking system including.   The system of claim 1, wherein the distortion processing system (100) includes a virtual electromagnetic tracker (400) that simulates the effect of distortion characteristics on tracking. A tracking analysis unit (120) for analyzing the tracking operation of the instrument (110);
A tracking correction unit (130) that corrects the tracking behavior of the instrument (110);
A distortion processing system (100) comprising:   The system (100) of claim 3, wherein the tracking analysis unit (120) generates at least one of a map and a model of strain characteristics of the instrument (110). An object simulation module (410) for simulating an object to be tracked;
A simulation toolset (420) capable of analyzing at least one distortion characteristic of the object and generating distortion information of the object based on the at least one distortion characteristic;
A virtual tracking system (400) comprising:   The system (400) of claim 5, wherein the simulation toolset (420) further comprises an accuracy module (430) for measuring the accuracy of the simulated tracking of the object.   The system (400) of claim 5, wherein the simulation toolset (420) further comprises a distortion detection module (440) that determines the effect of distortion by the object on tracking accuracy.   The system (400) of claim 5, wherein the simulation toolset (420) further comprises a distortion modeling module (450) that generates an electromagnetic model for evaluating the effect of distortion of the object on a tracker environment.   The system (400) of claim 5, wherein the simulation toolset (420) further comprises a distortion correction module (460) that improves the distortion tolerance of the electromagnetic tracker. A method (200) for distortion analysis in an electromagnetic tracking system, comprising:
Measuring the tracking behavior of the object (220);
Analyzing the tracking behavior of the object to determine the effect of distortion;
A method (200) comprising:   The method (200) of claim 10, wherein the analyzing further comprises generating at least one of a field map and a model for analyzing the tracking behavior of the object to determine the effect of the distortion.

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